Dust collectors, also known as bag houses, are generally used to filter particulate material from industrial effluent or off-gas. Once filtered, the cleaned off-gas can be vented to the atmosphere or recycled. Such a bag house dust collector structure generally includes one or more flexible filter banks supported within a cabinet or similar structure. In such a filter cabinet and bank, the filter bag is generally secured within the cabinet and maintained in a position such that effluent efficiently passes through the bag thereby removing entrained particulates. The filter bag, secured within the cabinet, is typically supported by a structure that separates the upstream and downstream air and supports the filter bag to maintain efficient operation.
More specifically, in a so-called “baghouse filter”, particulate material is removed from a gaseous stream as the stream is directed through the filter media. In a typical application, the filter media has a generally sleeve-like tubular configuration, with gas flow arranged so as to deposit the particles being filtered on the exterior of the sleeve. In this type of application, the filter media is periodically cleaned by subjecting the media to a pulsed reverse-flow, which acts to dislodge the filtered particulate material from the exterior of the sleeve for collection in the lower portion of the baghouse filter structure. U.S. Pat. No. 4,983,434 illustrates a baghouse filter structure and a prior art filter laminate.
The separation of particulate impurities from industrial fluid streams is often accomplished using fabric filters. These textile based filter media remove particulate from the fluids. When the resistance to flow or pressure drop through the textile caused by accumulation of particulate on the filter becomes significant, the filter must be cleaned, and the particulate cake removed.
It is common in the industrial filtration market to characterize the type of filter bag by the method of cleaning. The most common types of cleaning techniques are reverse air, shaker and pulse jet. Reverse air and shaker techniques are considered low energy cleaning techniques.
The reverse air technique is a gentle backwash of air on a filter bag which collects dust on the interior. The back wash collapses the bag and fractures dust cake which exits the bottom of the bag to a hopper.
Shaker mechanisms clean filter cake that collects on the inside of a bag as well. The top of the bag is attached to an oscillating arm which creates a sinusoidal wave in the bag to dislodge the dust cake.
Pulse jet cleaning techniques employs a short pulse of compressed air that enters the interior top portion of the filter tube. As the pulse cleaning air passes through the tube venturi it aspirates secondary air and the resulting air mass violently expands the bag and casts off the collected dust cake. The bag will typically snap right back to the cage support and go right back into service collecting particulate.
Of the three cleaning techniques the pulse jet is the most stressful on the filter media. However, in recent years industrial process engineers have increasingly selected pulse jet baghouses.
The need for high temperature (up to 200° C.), thermally stable, chemically resistant filter media in baghouses narrows the choice of filter media to only a few viable candidates for pulse jet applications. Common high temperature textiles comprise polytetrafluoroethylene (PTFE), fiberglass, or polyimides (polyimides are stable for continuous use to 260° C.). When the effect of high temperature is combined with the effect of oxidizing agents, acids or bases, there is a tendency for fiberglass and polyimide media to fail prematurely. Thus, there is a preference for using PTFE. Commercially available PTFE fabrics are supported needlefelts of PTFE fiber. These felts usually weight from 20-26 oz/yd2 and are reinforced with a multifilament woven scrim (4-6 oz/yd2). The felts are made up of staple fibers, (usually 6.7 denier/filament, or 7.4 dtex/filament) and 2-6 inches in length. This product works similarly to many other felted media in that a primary dust cake “seasons” the bag. This seasoning, sometimes called in-depth filtration, causes the media to filter more efficiently but has a drawback in that the pressure drop increases across the media during use. Eventually the bag will blind or clog and the bags will have to be washed or replaced. In general, the media suffers from low filtration efficiency, blinding and dimensional instability (shrinkage) at high temperatures.
Another type of structure designed for high temperatures is described in U.S. Pat. No. 5,171,339. A bag filter is disclosed that comprises a bag retainer clothed in a filter bag. The cloth of said filter bag comprises a laminate of a felt of poly(m-phenylene isophthalamide), polyester or polyphenylenesulfide fibers having a thin nonwoven fabric of poly(p-phenylene terephthalamide) fibers needled thereto, the poly(p-phenylene terephthalamide) fabric being positioned at the surface of the filter bag first exposed to the hot particle laden gas stream. The poly(p-phenylene terephthalamide) fabric can have a basis weight of from 1 to 2 oz/yd2.
A two layer product of porous expanded PTFE membrane (ePTFE) laminated to woven porous expanded PTFE fiber fabric has also been used. Commercial success of this product has not been realized due to several reasons, but primarily due to the woven fiber fabric backing not wearing well on the pulse jet cage supports. The woven yarns slide on themselves and create excessive stress on the membrane, resulting in membrane cracks.
Nonwoven fabrics have been advantageously employed for manufacture of filter media. Generally, nonwoven fabrics employed for this type of application have been entangled and integrated by mechanical needle-punching, sometimes referred to as “needle-felting”, which entails repeated insertion and withdrawal of barbed needles through a fibrous web structure. While this type of processing acts to integrate the fibrous structure and lend integrity thereto, the barbed needles inevitably shear large numbers of the constituent fibers, and undesirably create perforations in the fibrous structure, which act to compromise the integrity of the filter and can inhibit efficient filtration. Needle-punching can also be detrimental to the strength of the resultant fabric, requiring that a suitable nonwoven fabric have a higher basis weight in order to exhibit sufficient strength for filtration applications.
U.S. Pat. No. 4,556,601 to Kirayoglu discloses a hydroentangled, nonwoven fabric, which may be used as a heavy-duty gas filter. This filtration material however, cannot be subjected to a shrinkage operation. Exposure of the described fabric to a shrinkage operation is believed to have a negative effect on the physical performance of the filtration material.
U.S. Pat. No. 6,740,142 discloses nanofibers for use in baghouse filters. A flexible bag is at least partially covered by a layer having a basis weight of 0.005 to 2.0 grams per square meter (gsm) and a thickness of 0.1 to 3 microns. The layer comprises a polymeric fine fiber with a diameter of about 0.01 to about 0.5 micron, but is limited in basis weight due to the limitations of the process used to produce it. The limitation in basis weight of the layer in the '142 patent significantly reduces the lifetime of the filter medium and severely reduces the ability of the filter to survive cleaning cycles.